Osteoporosis is a major public health problem throughout the world. Although black women are believed to be at lower risk of osteoporotic fracture than white women, the risk of fracture in postmenopausal black women is still substantial. Blacks have consistently lower 25- hydroxy-vitamin D levels and an elevation in parathyroid hormone level compared to white women. These findings are consistent with vitamin D insufficiency, or in some, true deficiency. Vitamin D insufficiency has been implicated in bone loss with aging in the white female population. furthermore, in white elderly women and men, treatment with vitamin D has resulted in a lower rate of bone loss and fractures. The effect of vitamin D supplementation has never been evaluated in black women. We hypothesize that the presence of relative 25(OH)D insufficiency that exists among black postmenopausal women may exacerbate loss of bone among aging black women. To test this hypothesis, we propose to conduct a randomized clinical trial to compare vitamin D supplementation (1000 IU/ day) with placebo during a two year study in a population of 150 black postmenopausal (>5 years) women, who are shown to be 25(OH)D insufficient. We propose to determine whether supplemental vitamin D (1000 IU) improves mineral metabolism, increases calcium absorption, reduces bone turnover and reduces the rate of bone loss in black postmenopausal women. In addition, we intend to determine whether biochemical and bone density responses to vitamin D supplementation in black women are related to allelic variation in the vitamin D receptor. The fact that black women fracture less frequently than white woman and tend to have higher bone mass has virtually excluded this ethnic group from studies of osteoporosis treatment and prevention. However, the fracture rate in black women is still substantial. It is possible that a simple, safe nutritional supplement, vitamin D, could reduce the rate of bone loss and risk of osteoporotic fracture in black women.